424 PHYSIOLOGY OF THE INVERTEBRATA. 



The impregnation of the ova takes place within the body ; 

 and the ova, when laid, are enclosed in a cocoon which is 

 secreted by the integument. Although these animals are 

 hermaphrodites, copulation between two separate individuals 

 takes place. " The female copulatory organs are upon the 

 ventral surface of the anterior part of the body, and behind 

 the male organs — so that two individuals, by placing together 

 their anterior ventral surfaces in an inverse position, can be 

 mutually impregnated." (Von Siebold.) 



The ovum of Hirudo passes through a metamorphosis, the 

 mesoblast undergoing division into segments, which ultimately 

 give rise to the characteristic structure of this and other seg- 

 mented animals. 



The Oligochceta are hermaphrodites ; and the genital organs 

 are situated (like Hirudo) in the anterior part of the body. 

 In the fresh-water Oligochceta (Ifais and Tubi/ex), these organs 

 have no genital ducts, but the ova and spermatozoa are con- 

 veyed outwards by the nephridia, which are situated in those 

 segments of the body containing the genital glands. " In 

 Ifais and Chcetogaster, agamic multiplication occurs by the 

 development of posterior segments of the body into zooids, 

 which may remain associated in chains for some time, but 

 eventually become detached and assume the parental 

 form." 



In Lumbricus, the testes are two pairs of white sacs situated 

 on the posterior sides of the septa, separating the ninth and 

 tenth, and the tenth and eleventh segments. The spermatozoa 

 are not fully developed on leaving the testes, and they are 

 known in this condition as spermospores. The spermospores 

 are further developed by a process of budding, which takes 

 place in the vesiculas seminales (two pairs of reservoirs).. The 

 fully developed spermatozoa (Fig. ^6 a) are conveyed out- 

 wards by four ducts — the vasa deferentia ; but the. two vasa 

 deferentia on either side of the body unite, forming one duct, 

 which opens on the ventral side of the fifteenth segment. 

 The female genital organs consist of a pair of small ovaries 



